Surface acoustic wave devices are commonly used in wireless communications for meeting system-filtering requirements. Generally, surface acoustic wave (SAW) filters are manufactured by forming interdigital transducers on piezoelectric substrates such as Lithium Niobate, Lithium Tantalate, Quartz, or the like. Lithium Niobate and Lithium Tantalate have strong electromechanical coupling but generally suffer from high temperature drift, whereas Quartz has stable temperature characteristics but poor electromechanical coupling. For example, a 64° Y-cut, X-propagation Lithium Niobate typically exhibits 70–75 ppm/° C. of temperature drift and has a strong coupling of approximately 11%, while a 46° Y-cut in Lithium Tantalate has a temperature coefficient in the range of 40–60 ppm/° C. and coupling of about 9%. Quartz basically exhibits a zero linear temperature coefficient, but it has a very weak coupling coefficient of 0.15%. For RF SAW filters in wireless phone applications, by way of example, a strong coupling is a desirable substrate characteristic that results in a lower insertion loss and facilitates a filter design having a wider fractional bandwidth. As a result, Lithium Tantalate and Lithium Niobate are generally used in the manufacture of RF SAW filters. However, the relatively high temperature drift associated with these substrates typically precludes their use in SAW devices for certain applications that require a very high close-in rejection. As a result, SAW designers are constantly searching for ways to provide a SAW device on a substrate that can provide both strong electromechanical coupling and good temperature characteristics. A substrate with a combination of strong coupling and temperature stability will allow SAW designers to enhance the flexibility of their SAW device designs and extend SAW filter applications for the wireless phone handset, personal communications service, and PCS duplexer, by way of example, which have a very high close-in rejection requirements. Typically, PCS duplexers have been implemented using ceramic filter technology. While SAW filters are significantly smaller than comparable ceramic filters, the temperature drift of SAW devices on high coupling substrates such as Lithium Tantalate and Lithium Niobate make them undesirable for PCS duplexer application. Duplexers are dual band filters, typically connected directly after the antenna, providing the communications system with simultaneous filtering within the receive path and the transmit signal path. By being so very close to the antenna, key requirements for a PCS SAW duplexer are its power handling capability and low insertion loss along with steep skirt selectivity. Additionally, while Lithium Tantalate has a lower temperature coefficient as compared to Lithium Niobate, it is more pyro-sensitive. The pyroelectric effect is a phenomenon in which an electric field is built up due to a thermal change across the crystalline substrate. If not properly controlled, the pyroelectric effect may cause damage to the SAW device during the manufacturing process, thus reducing yield and reliability of the device.
In view of the aforementioned, numerous techniques have been proposed to improve the temperature characteristics of SAW filters fabricated on high coupling Lithium Niobate and Lithium Tantalate. By way of example, K. Yamanouchi et al. “SAW Properties of SiO2/128° Y-X LiNbO3 Structure Fabricated by Magnetron Sputtering Technique”, IEEE Transactions on Sonics and Ultrasonics, vol. SU-31, No.1, January 1984) have reported on the deposition of silicon dioxide film on top of the piezoelectric substrate of a 128°LiNbO and 36°YXLiTaO3 to improve the temperature characteristics of the SAW device. Yamanouchi showed that a SiO2 film thickness of 0.25 times the wavelength on a 128 LiNbO3 substrate could reduce its temperature coefficient to approximately zero. Parker et al. (“Stability of SAW Controlled Oscillators”, IEEE 1975 Ultrasonics Symposium Proceedings) demonstrated that the temperature coefficient of the SAW device on YZ LiTaO3 can be reduced to about zero when a layer of silicon dioxide with film thickness of approximately half that of the SAW wavelength is deposited on the substrate. However, these methods suffer from a major drawback in which the SAW properties depend upon the thickness of the SiO2 film. So while it is possible to demonstrate extremely good temperature performance, the SAW device is highly dispersive and thus prohibits the use of these techniques in production. Nakamura (“Effect of a Ferroelectric Inversion Layer on the Temperature Characteristics of SH-Type Surface Acoustic Waves on 36° Y-X LiTaO3 Substrates) discloses the formation of a polarized inversion layer on the surface of the 36°YX LiTaO3 to effectively change the elastic stiffness constants of the substrate thereby improving its temperature coefficient of delay. Taguchi et al. (U.S. Pat. No. 5,998,907) discloses a number of SAW composite structures combining the large thermal expansion of Lithium Tantalate and Lithium Niobate with that of a low thermal expansion substrate such as glass or silicon to achieve improved temperature characteristics of the composite device. In one embodiment, a 36°LiTaO3 substrate is bonded directly to a low expansion coefficient glass substrate. In yet another embodiment, a composite structure that comprises a thin film insulation layer of silicon dioxide of 1000-Angstrom is deposited on surrogate substrate of Si 36°LiTaO3 is then bonded to the substrate. This structure as disclosed by Taguchi has a major drawback in the performance of the filter passband characteristics. Since the thickness of the silicon dioxide is so very thin, there exists a direct capacitance coupling between the electrode pattern on the surface of the piezoelectric substrate with that of the Si layer, thus degrading the performance of the filter significantly. Thus, while the structure may provide better temperature characteristics, it suffers with a significant degradation in filter passband performance. This would result in higher insertion loss and a narrowing of the passband that renders them unsuitable for mobile applications.